1. Field of the Invention
The present invention relates to a rechargeable battery that comprises a jelly roll type electrode assembly that comprises two electrodes and separators for insulating the two electrodes.
2. Description of the Related Art
Rechargeable batteries are currently being developed for use in a variety of applications because of their many advantages including rechargeability, compact size, and large power storage capacity. For example, a nickel-metal hydride (Ni-MH) battery, a lithium (Li) battery, and a lithium ion battery are now available for use.
In most rechargeable batteries, the bare cells are formed by housing an electrode assembly in a cell container and sealing the cell container with a cover. The container may be a pouch or a metallic can that is made of iron, aluminum, or an aluminum alloy. An electrolyte for a rechargeable battery may be a solid polymer, an electrolyte impregnated gel type polymer, or a liquid polymer in which the electrolyte ions are mixed with a solvent.
The sealed bare cell is coupled with safety devices such as a positive temperature coefficient (PTC) device, a thermal fuse, and a protective circuit module (PCM) and other accessories, and then packaged in a separate case. Gaps that are present between the bare cell and the accessories are filled or coated with a plastic resin to provide a packaged appearance.
It is critical to manufacture a rechargeable battery that is capable of storing an abundant amount of electricity and is also compact and lightweight. In order to improve electric power capacity, the contact area between an electrolyte and an activation material must be increased. Typically, the activation material is coated on a wide area of an electrode plate to increase the surface area of the activation material and the ratio of the activation material that contributes to producing electrical energy.
The electrode plate that includes the activation material coating is wound in a roll shape to reduce the size of the rechargeable battery. In some cases a plurality of electrode plates are stacked alternately according to their polarities. The electrode plates that have the same polarity are coupled, thereby providing a positive electrode and a negative electrode of a battery. Also, a separator is interposed between the electrodes of opposite polarity to prevent an internal short circuit.
FIG. 1 is a perspective view illustrating an electrode of a jelly roll type electrode assembly in a typical lithium rechargeable battery.
FIG. 2 is a cross-sectional view illustrating a jelly roll that is formed by winding a first electrode, a second electrode, and separators.
Referring to FIG. 1, the electrode plate may be a rectangular shape with uncoated portions on which the activation material 131 is not coated at both ends. A tab 17 is attached to an uncoated portion in order to draw out electric charges that are stored in the electrode plate and transfer them to external connection wires. Typically, the electrode plate is formed by coating slurries on current collectors. The current collector may be an aluminum foil or mesh or a copper foil, and the slurries are formed by mixing the activation material with a resin or the like.
In a conventional jelly roll type electrode assembly as shown in FIG. 2, the tabs 16 and 17 for coupling the electrode plate to external devices are bonded to the current collectors and then the stack is wound. Therefore, when the electrode plates are fully wound with the tabs 16 and 17, a protruded part A is formed on the outer surface of the jelly roll type electrode assembly as shown in FIG. 2.
The size of the can for containing the electrode assembly matches the electrode assembly in order to minimize the volume of the resulting battery. In addition, the tabs 16 and 17 are very thin. For example, they may be about 0.1 mm thick. Although the tabs are very thin, when they are wound with the electrode plates, the part of the electrode plates that protrude on the outer surface of the jelly roll type electrode assembly is not negligible. When the electrode assembly is inserted into the can, the protruded part may become snagged on the opening of the can. Furthermore, when the electrode assembly is inserted into and assembled with the can, the protruded part may create pressure on the activation materials in the overlapped portions of the jelly roll. This may interfere with the electrochemical reaction and reduce the electric capacity of the battery. This problem is exacerbated when the protruded part is thick. Particularly, when the negative electrode is located in the center of the jelly roll, this problem becomes more serious.